The use of computers for both personal and business use has become so common that it is fair to say that almost all businesses, and many homes, have at least one computer or computer terminal. Computers are particularly prevalent in the office environment. Among the health issues which have become increasingly important as the use of computers has become more common is the need to promote proper posture while using the computer so as to both maintain working efficiency and minimize the occurrence of repetitive stress injuries. One basic principal which has been developed to address these issues is that the computer keyboard should be used while at a level considerably below the level of most desktops, so the user's wrists, when the keyboard is in use, are a "neutral" position; that is, the surface defined by the user's forearms and the top of his or her hands is flat. In addition to regulating the height at which the keyboard is used, it is also important for ergonomic purposes to control the angle which the keyboard shelf makes with the ground.
There have been a number of devices which have been developed for use in supporting keyboards (and associated computer accessories, such as a computer mouse) at a level below the surface of a desk while in use and underneath the desk when not in use. One such device is described in Smeenge, et al., U.S. Pat. No. 4,616,798, which discloses the use of a parallelogram linkage to connect a shelf for holding a keyboard to the underside of the desk surface. The overall system described by Smeenge permits a fair degree of flexibility in positioning the keyboard relative to the user and the desk. One disadvantage of Smeenge's system is that the parallelogram linkage used by the system is attached to the underside of the keyboard support shelf. Therefore, when the keyboard is positioned underneath the desk, the leg room available to the user between the bottom of the mechanism and the floor is limited.
McConnell, U.S. Pat. No. 5,257,767 attempted to address this deficiency of the Smeenge mechanism by using a non-parallelogram linkage to connect the keyboard shelf to the underside of the desk. This non-parallelogram linkage causes the front of the keyboard shelf to be angled upwards when the shelf is lowered relative to the desk top, marginally increasing the leg room available to the user when the keyboard is positioned beneath the desk. Like the Smeenge mechanism, the McConnell mechanism has its linkage attached to the bottom of the keyboard shelf, which limits the total leg room available to the user.